In the field of mechanical construction and in particular in the automotive field, the objective is to substantially reduce the weight of structures by using the least possible amount of metal. These structures, for example motor vehicle chassis, are obtained by assembling often complex components, performed by drawing.
In order to reduce the thickness of the metal used to produce these structures, while at the same time conserving their mechanical properties, steels with high mechanical characteristics are to be used. Grades of low alloy carbon steels with high mechanical characteristics are nowadays available, but are often associated with a very limited formability by deformation.
To clarify matters, we will differentiate these steels according to their elastic limit (EL) in the remainder of the description:                mild steels: EL<250 MPa;        steels with a high elastic limit (HEL):                    250 MPa<EL<600 MPa;                        steels with a very high elastic limit (VHEL):                    600 MPa<EL<1000 MPa;                        steels with an ultra-high elastic limit (UHEL):                    1000 MPa<EL<1500 MPa.                        
Typically, the steels to which this patent application is related have an elastic limit of between 400 and 1500 MPa. These steels are produced by bulk metallurgical processes that are known per se, which allow to offer steels at a cost price close to that of standard carbon steels. The advantage then lies in the fact that an appreciable lightening of the structure can be obtained. However, on account of their low formability and a sometimes poor weldability, these steels pose specific problems in terms of implementation, and in particular of assembly.
More particularly, the constituent parts of a same structure often have complex forms obtained by drawing processes which involve large deformations, and are thus incompatible with the low formability characteristics of these steels.
The process of mechanical hem crimping or the like is well known, for example for assembling parts such as ladders in the field of metal joinery. Thus, document U.S. Pat. No. 4,356,888 describes a structural joint for two parts, preferably made of malleable and deformable metal, such as aluminium. According to one particular embodiment, the first part has an elongated tongue and a short tongue. These two tongues define a cavity capable of receiving the second part at the level of a curved tab on a support such as a wire. When the two parts are pressed together using a suitable tool, the tongues and the tab are deformed and interpenetrate. The elongated tongue forms at least partially a circular loop around the tab, thus making any subsequent stripping impossible.
In the same field of application, document U.S. Pat. No. 3,854,185 proposes a process for forming a structural joint between two rigid parts, one having a flange with a protruding end on one face, and the other having an essentially circular groove. By pressing the two pieces against one another with sufficient force, the flange penetrates into the groove while becoming deformed. More specifically, the assembly is performed by securely LA fastening the flange to the groove, the flange forming a winding in said groove.
Document DE-C-385 642 proposes a machine for crimp assembling two plate metal parts in order to form a hollow body.
Document FR-A-2 321 962 proposes a process for crimp assembling a zinc part and a lead part in order to solve the sealing problems which arise in the field of construction roofing.